This invention relates generally to carrier sleeves receivable on flexographic printing cylinders, and more particularly to an improved carrier sleeve which is impermeable to air and which maintains its circular shape even when removed from the printing cylinder.
In the flexographic printing process, flexible printing plates formed of rubber or other elastomeric material are mounted on a printing cylinder, the plates being inked to make an impression on the paper sheets engaged thereby. In the conventional arrangement, once the flexographic printing plates are adhered to the printing cylinder, the cylinder is capable of running only one job. In order to run a different printing job, one must replace the cylinder with a cylinder having printing plates mounted thereon appropriate to this job. Hence one requires as many printing cylinders for the press as there are jobs to be run. The need for a multitude of printing cylinders adds substantially to operating expenses.
In order to facilitate the running of different printing jobs in succession on the same press by means of a common printing cylinder, a technique has been developed in which instead of adhering the plates directly onto the printing cylinder, the plates are supported on a carrier sleeve which is received by the cylinder and is removable therefrom. To run a different job with the same printing cylinder, it is merely necessary to replace the carrier sleeve with a sleeve carrying the appropriate flexographic plates.
This technique is disclosed in U.S. Pat. No. 3,146,709 of Bass et al., wherein use is made of a hollow printing cylinder having a pattern of apertures formed therein, such that when the interior chamber of the cylinder is coupled to a compressed air generator, air is caused to escape in small, high-velocity jets from the apertures. The apertured cylinder cooperates with a sleeve that is dimensionally stable in the longitudinal direction, the sleeve having a limited circumferential resilience so that it may be expanded slightly.
In order to mount the carrier sleeve on the apertured cylinder, the cylinder is set vertically on a suitable stand and its interior chamber is filled with compressed air. One end of the sleeve is manually slipped over the upper end of the cylinder, the air jets emerging from the apertures serving to slightly stretch the sleeve and to interpose a lubricating air film between the inner surface of the sleeve and the outer surface of the cylinder. This air film makes it possible to pull the sleeve down along the cylinder until it is properly in place thereon. At this point, the air is cut off and the sleeve, which is no longer subject to dilation, contracts to its normal diameter into tight frictional contact with the cylinder.
After the job is completed on the press, the cylinder and sleeve assembly may be returned to the stand. The air pressure is again turned on to slightly dilate the sleeve, making it possible to slide the sleeve off the cylinder, and to replace the removed sleeve with a new sleeve appropriate to the next run.
The Bass et al. patent discloses a sleeve made from a sheet of polyester film in combination with a helically-wound paper tape. Because of certain practical drawbacks inherent in the use of paper, such sleeves are not presently in commercial use. Carrier sleeves that are currently employed in conjunction with flexographic printing cylinders are fabricated entirely of synthetic plastic film material.
Existing carrier sleeves are formed from a rectangular sheet of film material whose opposite edges are adjoined to define an inner tube. A strip of plastic tape having an adhesive layer thereon is helically wound about the inner tube to define a first helix thereabout. Wound in the opposite hand about the first helix is a second strip of plastic tape having an adhesive layer thereon to define a second helix.
Since the adjoining edges of the inner tube necessarily have a slight spacing or air gap therebetween to avoid overlap of the edges that would produce a sleeve of uneven thickness, the wall of the inner tube is not entirely impermeable to air. The first helix, though wound about the inner tube, does not completely close the longitudinal gap in the inner tube in that the slight spaces between adjacent convolutions of the helix give rise to leakage paths. And while the convolutions of the second helix intersect those of the first helix and partly close the spaces between the convolutions of the first helix, the spaces between the convolutions of the second helix produce leakage openings which, though quite small, are nevertheless permeable to air and degrade the air film created between the sleeve and the printing cylinder.
Thus with existing types of sleeves, the presence of leakage paths in the sleeve somewhat reduces the effectiveness of the lubricating air film produced by the air jets emerging from the cylinder. But a more serious drawback arising from such leakage is that the resultant air jets impinge on the adhesive holding the printing plates to the sleeve, causing bubbles to form therein, which weaken the bond and loosen the plates.
Another drawback is a lack of cylindricity in existing types of sleeves. Because the inner tube of this sleeve is formed of a rectangular sheet of plastic film bent into tubular form, the sleeve, when free of the printing cylinder, assumes a "tear-drop" rather than a circular shape. Because of this non-circular shape, there is a tendency for flexible printing plates mounted on the sleeve to lift up at the apex of the tear-drop. Moreover, the lack of cylindricity makes it more difficult to slide the sleeve onto the perfectly circular printing cylinder.
Another limitation of existing sleeves which is imposed by the inner tube is that one cannot readily manufacture any length and diameter of carrier sleeve, for one must cut the sleeve material of the inner tube into rectangles or parallelograms to fit a specific diameter and length.